Rare earth elements, also known as the lanthanide family of elements, are commercially valuable metals that are generally present in low abundance in commercially accessible ores, and often are often found with undesirable contaminating elements (for example, thorium). In a typical process for isolation of a member of the lanthanide family, hot sodium hydroxide at high concentrations is used to generate soluble lanthanide hydroxide and thorium hydroxide from ore. The mixture of hydroxides is then treated with hydrochloric acid to generate lanthanide chlorides, which are soluble and remain in solution, and a sludge of thorium hydroxide (which has reduced solubility at the altered pH). Unfortunately, this process can leave significant amounts of thorium in solution following acid treatment. Since this element is radioactive significant further refinement steps are necessary to assure its removal, adding significantly to processing costs. In addition, the use of a strong base at an elevated temperature both presents a hazard to workers and requires the use of specialized equipment. Thus there is a pressing need for efficient, effective, and scalable methods for the isolation of rare earth elements at high purity.
Numerous approaches have been devised to attempt to address these issues. Hydrometallurgy, or the extraction of metals from ores through treatment with lixiviant solutions (i.e. lixiviants) is one approach that has been used successfully for the isolation of metals from a variety of minerals and other sources. In typical hydrometallurgical processes ore is crushed or pulverized to increase surface area prior to exposure to a lixiviant, which contains compounds that render the metal soluble in the solution and leave behind undesirable contaminants. Following collection of the solution the metal can be recovered from the solution by various means, such as by electrodeposition or by precipitation from the solution. Commercial development of hydrometallurgical processes, however, is often hindered by the expenses involved in production and use of the lixiviant, efficient recovery of the desired metal, and difficulties in adapting current commercial plants.
In an approach disclosed in U.S. Pat. No. 5,939,034 (to Virnig and Michael), metals are solubilized in an aqueous lixiviant containing ammoniacal thiosulfate and extracted into an immiscible organic phase containing guanidyl or quaternary amine compounds. All publications identified herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
Metals are then recovered from the organic phase by electroplating. A similar approach is disclosed in U.S. Pat. No. 6,951,960 (to Perraud) in which metals are extracted from an aqueous phase into an organic phase that contains an amine chloride. The organic phase is then contacted with a chloride-free aqueous phase that extracts metal chlorides from the organic phase. Amines are then regenerated in the organic phase by exposure to aqueous hydrochloric acid. Such approaches, however, are hindered by the use of volatile amines in the lixiviant, and necessarily involve the use of expensive and potentially toxic organic solvents.
In a related approach, European Patent Application No. EP1309392 (to Kocherginsky and Grischenko) discloses a membrane-based method in which copper is initially complexed with ammonia or organic amines. The copper:ammonia complexes are captured in an organic phase contained within the pores of a porous membrane, and the copper is transferred to an extracting agent held on the opposing side of the membrane. While such an approach minimizes the use of organic solvent, it requires the use of complex equipment that is not readily adaptable to current processing facilities. In addition, the capacity of such a process is necessarily limited by the available surface area of the membrane.
Kodama et al. (Energy 33(2008), 776-784) disclose a method for CO2 capture using calcium silicate (2CaO.SiO2) and a solution of ammonium chloride (NH4Cl). This reaction forms soluble calcium chloride (CaCl2), which is reacted with carbon dioxide (CO2) under alkaline conditions to form insoluble calcium carbonate (CaCO3) that captures CO2 while releasing chloride ions (Cl−). Japanese Patent Application No. 2005097072 (to Katsunori and Tateaki) discloses a similar method for CO2 capture, in which ammonium chloride (NH4Cl) is dissociated into ammonia gas (NH3) and hydrochloric acid (HCl), the HCl being utilized to generate calcium chloride (CaCl2) that is mixed with ammonium hydroxide (NH4OH) for CO2 capture. Kodama et al. and Katsunori and Tateaki, however, fail to recognize hydrometallurgical applications of such reactions, and rare earth elements are not considered. In addition, the loss of highly volatile ammonia during processing results in both inefficiencies and the need for specialized equipment to reduce environmental impact.
International Application WO 2012/055750 (to Tavakkoli et al.) discloses a method for purifying calcium carbonate (CaCO3), in which CaCO3 from high content sources is converted to calcium oxide (CaO) by calcination. The resulting CaO is treated with an ammonium chloride (NH4Cl) solution to produce calcium chloride (CaCl2), which is subsequently reacted with high purity carbon dioxide (CO2) to produce CaCO3 and NH4Cl. High purity CaCO3 is subsequently recovered from the solution by crystallization using seed crystals. Tavakkoli et al., however, does not consider rare earth elements, and it is not clear if such an approach can be used with low content or highly contaminated starting materials. In addition, utilization on a large scale would require capturing or containing the highly volatile ammonia gas that results from such a process.
Attempts have also been made to recover specific rare earth elements from mixed solutions. U.S. Pat. No. 9,115,419, to Laksmanan et al, describes a process in which a number of rare earth metal species and iron are extracted from an ore using an acidic MgCl2 lixiviant, with specific rare earth species being selectively removed from the resulting solution by extraction with various organic solvents. Such solvents, however, pose a significant environmental hazard, and it is not clear if specific rare earth elements can be isolated in a sequential manner. Similarly, United States Patent Application Publication No. 2013/0309150, to Takur, describes processes for recovery or rare earth elements from waste phosphors by extraction using a strong acid lixiviant, followed by selective recovery of groups of rare earth elements and certain rare earth elements by organic solvent extraction. In addition to the environmental issues posed by the use of such solvents, however, it is not clear if the proposed method can be utilized with more typical raw materials that include a greater number and wider variety of contaminating materials.
Thus, there is still a need for scalable hydrometallurgical methods that provides simple, economical, and selective isolation of rare earth elements.